This invention relates generally to an electrophotographic printing machine, and more particularly concerns an improved apparatus for dispensing toner particles.
In electrophotographic printing, a photoconductive member is charged to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image of an original document being reproduced. Exposure of the sensitized photoconductive surface discharges the charge selectively. This records an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document being reproduced. Development of the electrostatic latent image recorded on the photoconductive surface is achieved by bringing a developer material into contact therewith. Typical developer materials comprise dyed or colored heat settable plastic powders, known in the art as toner particles, which are mixed with coarser carrier granules, e.g. ferromagnetic granules. The carrier granules have the toner particles adhering to and coating the surface thereof due to the triboelectric attraction therebetween. During development, the carrier granules with the toner particles adhering thereto contact the electrostatic latent image. The electrostatic latent image has a charge of opposite polarity to the charge on the toner particles. The toner particles are pulled away from the carrier granules by the latent image and deposited thereon to form a powder image. The partially denuded carrier granules return to the developer sump. As toner powder images are formed, additional toner particles must be furnished to the developer mixture in proportion to the amount of toner deposited on the latent images.
If prints made by the electrophotographic printing process have heavy deposits of toner particles in image areas of good contrast and non-image areas having a gray background, the toner concentration in the developer mixture is too great. Contrawise, if the prints have low contrast images, the quantity of toner particles in the developer mixture is insufficient. In order to continually obtain prints of good quality, the quantity of toner particles in the developer mixture must be held reasonably constant. This is achieved by the addition of toner particles to the developer mixture in proportion to the amount of toner particles deposited on the latent images.
Various methods have been devised for introducing additional toner particles to the developer mixture. However, these methods generally dump large amounts of toner particles into the mixture and do not, necessarily, uniformly disseminate them thereto. Many of the techniques employed to discharge toner particles employ gravity and control the particle flow rate from the storage reservoir. Some machine configurations do not readily permit this type of dispensing due to physical interferences of the required location of the storage reservoir with other machine components. To solve this problem, the storage reservoir is frequently located remotely in the machine and a transport system for moving the toner particles from the storage reservoir to the developer housing is provided. Uniform dispensing in this type of system is frequently difficult to achieve, particularly when the flow characteristics of the toner particles may vary, e.g. with changes in relative humidity.
Accordingly, it is a primary object of the present invention to improve the dispensing of toner particles to a developer mixture.